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Old 12-16-2020, 11:48 AM   #11 (permalink)
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I think the confusion arises because the video omits the large vortex that forms in a diffuser that only has side strakes, maybe because that vortex is much smaller or irrelevant in a diffuser with many strakes. Or maybe it is beyond the scope of the video

"The video specifically states that the strakes and their tip vortices separate the flow coming off the rear wheels from the rest of the diffuser, at 7:44." They probably do, maybe that is not the only benefit though.

"He also says several times (and takes as his initial premise) that the vortices rolling off the strake edges stay at those edges," Well, there is a vortex that stays at the edge, he omits the massive vortex that forms though (see first paragraph)


"The video also asserts that the vortices rolling off an L-shaped vane occur at the tip of the horizontal portion, but images in the article show the opposite--the vortices roll up the opposite side from the L."
as I said in the previous post "The last image (in the article) shows a vortex at the edge of the L and one inside the diffuser, the video just mentions the vortex at the edge of the L." The picture shows two vortices, the huge one in the diffuser, and the small one at the edge of the L like in the video. Presumably the big vortex was beyond the scope of the video.

"Finally, the article ends with the explanation that fences in the middle of diffusers are there specifically to generate vortices to improve generation of downforce, whereas the guy in the video says that the fences are there to separate wheel flow from the inward part of the diffuser and that this improves downforce by not messing up the flow, essentially (last two minutes of the video)." I don't know, I think it is a bit of both, tyre squirt probably affects a diffuser, and the generation of vortices stop the squirt being as much. The vortices (as mentioned in the article) improve downforce generation, probably partly due to a sealing effect and probably partly blocking air ingress from tyre squirt.

"Two different explanations, but I don't know which one is correct. Perhaps neither." I think they are both correct. But I think both are credible, if a bit simplified and too definite, it is probably something in the middle.

I am no expert and that article was a good read. Hopefully somebody who knows more than me can help.

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Old 12-16-2020, 01:11 PM   #12 (permalink)
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And to add to the discussion, I think that in road cars, vortices don't have a lot to do with how diffusers work! My tuft testing and pitot tube measurements seem to confirm that.

Diffuser effectiveness in reducing lift can on road cars can easily be explained by looking at the vector force direction as the attached flow curves upwards.

Think of the same process occurring, but this time on the upper surface of the car, eg over a curved rear hatch (so of course giving lift not downforce). Now make that upper surface have a massive pressure drop, such that flow wraps around from the side to the top surface, so creating strong vortices. Clearly lift will increase a lot. But so will drag! Drag doesn't matter much in racing cars, but concerns us.

This is one reason I don't like much looking at race car aero when talking about road cars.
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Old 12-16-2020, 01:49 PM   #13 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
Think of the same process occurring, but this time on the upper surface of the car, eg over a curved rear hatch (so of course giving lift not downforce). Now make that upper surface have a massive pressure drop, such that flow wraps around from the side to the top surface, so creating strong vortices. Clearly lift will increase a lot. But so will drag! Drag doesn't matter much in racing cars, but concerns us.
Probably so! The diffuser angles in the article are much higher than typical road cars, up to 20 degrees, and at much lower ride heights.

Quote:
Originally Posted by AeroMcAeroFace View Post
I think the confusion arises because the video omits the large vortex that forms in a diffuser that only has side strakes, maybe because that vortex is much smaller or irrelevant in a diffuser with many strakes. Or maybe it is beyond the scope of the video

"The video specifically states that the strakes and their tip vortices separate the flow coming off the rear wheels from the rest of the diffuser, at 7:44." They probably do, maybe that is not the only benefit though.

"He also says several times (and takes as his initial premise) that the vortices rolling off the strake edges stay at those edges," Well, there is a vortex that stays at the edge, he omits the massive vortex that forms though (see first paragraph)
But if it's the large vortex that is responsible for the downforce generated by these diffusers, wouldn't it be important to mention that? He says he has another video on diffusers, maybe he does there.

Quote:
Originally Posted by AeroMcAeroFace View Post
"The video also asserts that the vortices rolling off an L-shaped vane occur at the tip of the horizontal portion, but images in the article show the opposite--the vortices roll up the opposite side from the L." as I said in the previous post "The last image (in the article) shows a vortex at the edge of the L and one inside the diffuser, the video just mentions the vortex at the edge of the L." The picture shows two vortices, the huge one in the diffuser, and the small one at the edge of the L like in the video. Presumably the big vortex was beyond the scope of the video.
Hmm, I'm not seeing the same as you. I see clear rotation on the inside of that fence, but high-velocity, non-rotating flow on the outside edge of the L:

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Old 12-16-2020, 04:29 PM   #14 (permalink)
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With regards to that picture I think that the low pressure is a very tight vortex with a high spin rate hence the very low pressure. I am no vortex expert though. Annoyingly I can't do CFD and there are no better pictures that I can find. To me the flow lines seem to be circular there, but maybe I am imagining it, the picture is so bad.

In the below image, there is low pressure under the wing, imagine rotating the image along the x axis, such that the vertical axis Z is facing the same way as the Y axis is now. The secondary vortex shown, I believe is the small area of blue on the CFD


"I think that in road cars, vortices don't have a lot to do with how diffusers work!" I would agree with you, I have read very little about vortices in diffusers.

"if it's the large vortex that is responsible for the downforce generated by these diffusers" I don't actually think that it is, I don't quite understand it, so I may be wrong, but I don't think you need large vortices to get downforce.

I actually read a whole doctoral thesis a few months ago about diffusers on Saab cars and there was no mention at all of vortices within or at the edge of the diffuser.

I think vortices are only really relevant with low ground clearances and high pumping ratios. A very low car at 15cm ground clearance would need a diffuser that is 75cm high to have a 5:1 pumping/area ratio. And realistically there aren't any road cars that would have a diffuser that big.
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Old 12-18-2020, 01:21 PM   #15 (permalink)
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Quote:
Originally Posted by Cd View Post
Great video.
As Stubby said, knowing what doesn't work is just as important as knowing what does !

You mention strakes on your diffuser didn't work.
They didn't produce any additional downforce.

But i added strakes to mine in the hope that it would staighten out airflow.
That, and I think they look cool ( which means even if ineffective, I plan to still use them unless they actually create drag.

Would strakes help guide air that is going turbulent ? ( It's related the questions I asked before about using fences on cars. )
Some messengers have reported that the outboard strakes help isolate the diffuser from rear wheelhouse turbulence effects. Then go on to mention additional buffering from crosswind and gust effects. A safety precaution on the race course, or rain-slick Autobahn.

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